From an SEO point of view, Google says that it treats sub-domains and sub-folders the same way, but some SEO experts believe that sub-domains will be considered new and separated from the main website, therefore, going for a sub-folder is much better.
A video by Google Webmasters explains which one is the most beneficial for SEO:
That number was not how many challenges a user had completed, but how many brownie points a user had earned. Brownie points could be earned not only by completing challenges, but by receiving “thanks” in gitter.
This confusing user experience is on of the reasons we removed the feature.
And what of chapters and study groups (communities?)? Will these remain part of the whole suite?..
I actually co-hosted an event today for our city chapter, and went looking for the chapter listings/index along with a search for the entry point of how a person interested in starting a chapter or study group would get started. I could not find any related mention of navigation for this, until I turned to google to search for it.